Select to expand quote
I am just trying to understand the basics of 12 Volt battery use.
I thought this article as a starter was very good.
Please feel free to expand on it. i.e. your power sources and what you run off them.
A GUIDE TO ELECTRICAL BASICS
ELECTRICAL BASICSVOLTS, AMPS, WATTS AND ALL THOSE OTHER THINGSThis information is on a need-to-know basis, and if you plan on getting the best out of your 12Volt stuff - you need to know!Fortunately the 12Volt equipment that we use for camping and caravanning is DC (Direct Current) and that simplifies things a lot.DC SYSTEMSFirstly, DC voltage and current behave nicely, so you can add and subtract them, and even multiply them - in 240 Volt AC circuits you can't, well not easily, but that's not our concern, we will be talking about 12Volt stuff. When we get into inverters we'll need a little bit of AC knowledge but we'll leave that for now.Secondly, in DC circuits, watts are watts - I know that may not sound terribly encouraging, but in AC circuits power appears in all sorts of forms - real, apparent, reactive, and even imaginary power. So, in 12Volt DC systems we will only have to deal with power measured in Watts. All good!VOLTAGEThe most basic electrical quantity is probably Voltage, measured in Volts. Helpfully, the symbol for Voltage and Volts is V - this may also not seem very exciting but wait till we get to current. Voltage is a measure of the electrical force that the electrical system can exert on the equipment it is supplying. 12 Volts is classified as low-voltage and as we will see when we begin to calculate volt-drop, this is a distinct limitation compared to 240V systems. Anyway, we have ways to get around that, so for the moment we'll just note it as a limitation. Voltage can be measured quite simply and inexpensively with a voltmeter, or by using a multimeter set to measure voltage.CURRENTThe next electrical quantity we're usually interested in is current, measured in Amps. The symbol for current is capital-letter I and for Amps we use the letter A. So if we had 5 Amps of current being delivered by a solar panel this could be written as ISOLAR = 5 A. We have the French to thank for the letter I - for intensit? de courant, meaning current intensity. Anyway, current is the flow of electrons through a circuit - the more electrons per second, the more the current. To use a water-analogy, current would be equivalent to the water-flow, whereas voltage would be the pressure behind that flow. To measure Amps we need an Ammeter - who would have thought? - and this needs to be placed in the flow of current. So that's the theory, now how does that work in practice?MEASURING CURRENT
Break the circuit to insert the Ammeter
Well if we look at the circuit here which shows a simple solar-regulator-battery circuit, then to measure how much current I'm getting into the battery I need to somehow "break-into" the circuit to make the measurement. Again, if we use a water-analogy then to measure the flow we'd need to break the pipe somewhere and insert a flow-meter. It's the same in our 12 Volt system - we break the circuit and insert an Ammeter into the circuit as shown in the diagram. On paper that's easily done, but what happens if the battery is under the bed, the regulator is in the boot, and the solar panel on the roof of the caravan?!
Different Positions measure Different Currents
Well, in principle it's the same - we need to break the circuit somewhere to measure the current. It also depends what current we want to measure. If we now bring in a few more practicalities, like fridges & lights that are drawing current from the battery, then we have a choice of which current we want to measure.If we just want to measure the amount of current coming in from the panels, then we place the Ammeter in position-1. If we want to measure the current going to the fridge and lights, we place it in position-2. And if we want to measure the current going into-and-out-of the battery, then we'd put it in position-3. If this all becomes a little brain-scrambling then escaping back to the water-analogy usually helps.So those are our two main electrical quantities - voltage and current - measured in units of Volts and Amps. It's worth pointing out the somewhat obvious, namely that electricity can't be seen, so meters are our only way of knowing what's going on. In a water system we can look at how full the tank is and get a rough idea, and looking at the water coming out of the pipe will give us an idea of the flow. In electrical circuits we're blind - meters are our only way of getting a fix on what's happening.SOLAR REGULATORSOur circuit included a solar regulator, and many of these have meters built in, allowing us to see the battery voltage, the solar current coming in, and the current going out to the loads like fridges and lights. The more sophisticated ones will also measure things like Amp-hours coming in and going out, and using that they work out a guesstimate of the energy left in your battery (as a percentage). There's more on this in the blog post about Solar Regulators.WATTS AND AMP-HOURSNow that we've started to mention Amp-hours and other things, let's progress to some more complex electrical quantities - we'll start with Power (P), which is measured in Watts (W). Closely related is Energy, measured in Amp-hours (Ah) and it may be just as well to try and untangle these two units right now. In terms of formulas, both are a simple multiplication. Power = Voltage x Current, and Energy = Current x Time, or using their units we have Watts = Volts x Amps, and Amp-hours = Amps x hours.POWER AND ENERGYTo explore the difference between power and energy, let's say we have a 12 Volt battery - I think most people reading this article would be familiar with that. Without getting into questions of deep-cycle or starting batteries, we'll just consider two battery uses: first, starting a vehicle, and second, running a 12 Volt fridge for a day.Starting the engine typically requires hundreds of Amps but it's for a very short time - running a 40-litre fridge for 24 hours usually averages a current of about 1.5 Amps, so the current is tiny by comparison, but it keeps on going. If we calculate the energy and power in these two examples, hopefully that'll give us an idea of how energy and power relate.THE STARTERSo if the starting current was 200 Amps and the time taken to start was 10 seconds[*], then to get the energy, we need to determine the Amp-hours. The current is easy, that's 200 Amps - now we need to get those seconds into hours - the number 60 springs to mind, right? So it's (10sec/60) to get minutes = 0.167 minutes, and to get hours we divide by 60 again, which gives us 0.00278 hours.So to get the energy in Amp-hours we just multiply the Amps by the hours, so 200A x 0.00278hrs = 0.556 Amp-hours.THE FRIDGENow how much energy does our fridge use? Well it's again the current multiplied by the time, so we get 1.5 Amps x 24 hours = 36 Amp-hours. So now we can see that the starter actually uses way less energy than the fridge. That's of course if all goes well and we start in 10 seconds - we all know what happens if we don't start the engine that quickly, don't we?! But I digress.Back to our two scenarios - let's now compare them in terms of power. Over a day the fridge uses an average of 1.5 Amps, so the average power it uses will be 12 Volts x 1.5 Amps = 18 Watts. The starter on the other hand uses a lot more current so the power will be 12 Volts x 200 Amps = 2400 Watts = 2.4kW.So the fridge uses very little power but for a long time, while the starter uses heaps of power but for a very short while. Let's try and relate this back to our typical 12 Volt systems.BATTERIES AND AMP-HOURSAt the heart of virtually any 12 Volt system will be a battery. You will probably have noticed that deep-cycle batteries, the ones used to run fridges, lights, pumps, etc. are specified in terms of Amp-hours. So that means a 200Ah battery will store twice the energy of one specified as 100Ah. Working out how long the battery will last on our fridge above is simply the Amp-hours divided by the Amps, so on a 100 Ah battery our fridge should last (100Ah/1.5Amps) = 66.7 hours, so a bit less than 3 days.AVERAGE CURRENTWhile we're talking Amps and Amp-hours let's just clear up a common misconception - this occurs especially when talking about fridges which cycle on and off depending on how much cooling is needed. For instance in hot weather a fridge will stay on for longer and be off for shorter periods - this will increase the energy it draws from the 12 Volt system. If we average that energy draw over say 24 hours then we get the average current drawn - in our case above that was 36Ah over 24 hours, so an average current of 1.5 Amps.AMPS PER HOUR?In the example of our fridge some folks may be tempted to say that the fridge is drawing 1.5 Amps per hour - this statement makes no sense - it would be like saying that a boat was travelling at 11 knots per hour - ?!?What they might mean, is that for every hour that goes past, we are drawing 1.5Ah of energy from the battery - that would make sense. Or they could mean that the average current draw, averaged over an hour, is 1.5 Amps - that would also be correct. But Amps per hour makes no sense in this context.RESISTANCEThe only other electrical quantity that needs a mention is resistance, measured in Ohms, using symbols R and the Greek capital-letter omega ?. Resistance, voltage and current are inter-related by a simple but powerful equation known as Ohm's Law. A usefully compact form of this is a triangle with V at the top. In this form we can get the equation V=IR or R=V/I or I=V/R so we get 3 forms of Ohm's Law in one diagram. We'll use this when calculating volt-drop in cables, a very important thing to do in 12 Volt systems.So, that's about all we need in terms of electrical quantities and their units to get us through most 12Volt systems. That wasn't so hard, was it?[*] 10 seconds may be a bit pessimistic, but it makes the maths easier.
While we're on the subject of batteries, I have a question.
My boat has a house master switch and a start master like most others i guess. Normally the house system is sitting around about 12.8 - 13.1v SOC, and the start battery about the same. I habitually turn off the start master when not using the motor. I noticed the other night when I turned off the start master that the SOC of the start battery dropped rapidly to around 11.8-11.7v SOC (multimeter at the terminals after i cleaned them). Is this because it's connected somehow to the house system, and is being supported by it when on, but is actually stuffed? Does turning off the start master simply isolate that one battery?, cause when i switched it back on, the SOC jumped right back up to match the house. Also, there seems to be far too much wiring attached to the start battery as compared to the house system. I don't know where it all goes, although i can account for some of it (starter, windlass, lifting pump) Could this be the problem? Or an earth leak?
I have no idea how to troubleshoot this
, any suggestions would be much appreciated.
Select to expand quoteAzureF305 said..
While we're on the subject of batteries, I have a question.
My boat has a house master switch and a start master like most others i guess. Normally the house system is sitting around about 12.8 - 13.1v SOC, and the start battery about the same. I habitually turn off the start master when not using the motor. I noticed the other night when I turned off the start master that the SOC of the start battery dropped rapidly to around 11.8-11.7v SOC (multimeter at the terminals after i cleaned them). Is this because it's connected somehow to the house system, and is being supported by it when on, but is actually stuffed? Does turning off the start master simply isolate that one battery?, cause when i switched it back on, the SOC jumped right back up to match the house. Also, there seems to be far too much wiring attached to the start battery as compared to the house system. I don't know where it all goes, although i can account for some of it (starter, windlass, lifting pump) Could this be the problem? Or an earth leak?
I have no idea how to troubleshoot this, any suggestions would be much appreciated.
it depends on how your boat is wired up.
can you start your motor with the house master on and the start master switch off? if so then both batteries are probably connected to everything when they are on.
Cant start without the start master on, and will start with house switched off.
House master powers up the panel board and fridge etc.
I did notice that when i had the connections off the neg terminal on the start battery for cleaning, that the house panel went dead, i didn't think thats supposed to happen, but i put everything back the way it was in fear of mucking it up by changing it.
12 volt electrics is a black art. Good luck mate. Often the best way is to rip it all out and start again from the begining and make wiring diagrams as you go. Good winter job 
Select to expand quoteAzureF305 said..
While we're on the subject of batteries, I have a question.
My boat has a house master switch and a start master like most others i guess. Normally the house system is sitting around about 12.8 - 13.1v SOC, and the start battery about the same. I habitually turn off the start master when not using the motor. I noticed the other night when I turned off the start master that the SOC of the start battery dropped rapidly to around 11.8-11.7v SOC (multimeter at the terminals after i cleaned them). Is this because it's connected somehow to the house system, and is being supported by it when on, but is actually stuffed? Does turning off the start master simply isolate that one battery?, cause when i switched it back on, the SOC jumped right back up to match the house. Also, there seems to be far too much wiring attached to the start battery as compared to the house system. I don't know where it all goes, although i can account for some of it (starter, windlass, lifting pump) Could this be the problem? Or an earth leak?
I have no idea how to troubleshoot this, any suggestions would be much appreciated.
HI Azure,
AS Fish mentioned, it all depends on how it is wired. The older the boat the more bodgy quick solutions have been overlaid and not routed to the right spot, most often due to convenience.
So in general;
- 11.8V on a battery is bad (I'm ruling out the charging system as your house is fine) which sounds like you lost a cell in your starting battery. Can you see any deformation of the battery? (I just replaced mine for the same reason, and when I pulled it out there was a noticeable bulge on one end from the bad cell) . To test the battery;
--- Disconnect the terminals to the boat cabling.
--- Get something that will draw power, for example I used a little portable air pump for the RIB, connect it to the battery.
--- Have your multimeter connected to the battery.
--- Turn on the load device. If the battery is bad, you will see an immediate drop below 12.5V ( Mine went straight to 11V.) Ergo: it's toast.
- When you turn on your starter battery and it jumps back up to 12.8-13.1, this says to me the starting battery is connecting to an active charging system, or it is connecting to the house battery. Once a cell dies it will happily steal power from anywhere to bring itself back to normal, but it doesn't hold the charge anyway so its being parasitic all the time. You need to follow the wiring to work that one out I think.
- My starting battery has only the normal starter motor cables plus one small positive terminal, nothing else. I am down the boat tonight, I'll see if I can work out what the little + one is, at least that might give you a reference. My windlass, pump, everything else connects to the house battery bank. I have to have the engine running before I can lift the keel or use the windlass, but that's because of the high current draw, it all still connects to the house.
- If you disconnect the starting battery and your house panel dies, someone has cheated and there is a connection from your house panel to your starting battery.
Hope that helps mate, I'll report back on my unknown starting battery + terminal and cable.
Select to expand quoteAzureF305 said..
Cant start without the start master on, and will start with house switched off.
House master powers up the panel board and fridge etc.
I did notice that when i had the connections off the neg terminal on the start battery for cleaning, that the house panel went dead, i didn't think thats supposed to happen, but i put everything back the way it was in fear of mucking it up by changing it.
Without knowing your wiring it's hard to say what's going on. It does sound like one of your battery banks is healthier than the other and when you are turning both your switches on you are paralleling your batteries, your good battery pulling the volts up on your meter, that also means your good battery can discharge into the bad battery (not ideal).
If you made your measurements when the engine is running or when batteries are on load that will affect your results as well. For example an alternator charging voltage is about 13.8-14.2 Volts.
Thanks Shaggy and Seamonkey - I think you've connected to a few dots for me. All measurements were made at night with everything turned off, so no charging or load. Im now thinking that its a bit of bodgey wiring in combination with a bad starter battery. The house system is rock solid and checks out fine, so I'm looking at the starter as the fail. Ill do the testing tonight, and the physical sighting tomorrow with better light. I think only one wire from the start neg connected to the house so I can re-run that one to the house- that would solve that problem, and I could probably work out where the other ones are coming from, but i think my biggest issue will be sorting this -
- or do i just sort out the batteries and leave this alone? (pic should be portrait, sorry)
I'm guessing it's the lifting pump but why all the wiring?
Azure, I agree start battery likely to be suss. what charging system/s do you have ? Alternator only - panels - wind ? And I'm guessing both 12v systems ?
HI Azure,
Yep, keel lift pump. Having just been through the same thing, I took a photo and marked it up so I could remember what I was doing when I pulled it down , I labelled the wiring in the pic following the three main circuits;
- Main power circuit
- Thermistor circuit
- Control button circuit.
Took me a while to work it out, hopefully yours is similar and I can save you some wasted time !
Sorry Shaggy, I should have specified - I meant fuel lifting pump. But im still guessing
Select to expand quoteAzureF305 said..
Sorry Shaggy, I should have specified - I meant fuel lifting pump. But im still guessing
Doh! I was thinking of your lovely First 305, so assumed it was the lift keel pump you were referring to. My bad, sorry mate.
Hi Azure
Follow Shaggies advice.
What you have here is a solenoid contactor.
Nothing to do with fuel lifting pump even though it is mounted right next to the filter,
We use contactors when we want to switch on/off a high amperage load.
This is usually the starter motor or it can be a battery isolation switch.
The two heavy lines on the left and the right are the lines which are switched. (on yours , left = line in , Right = line out) I think.
The smaller lines at the front are the control circuit connections. One is positive and one is negative. 
As you can see you have a red wire and a black wire connected to the same control circuit lug. looks like someone ran out of one colour wire.
That screams bodgy job somewhere along the line.
These solenoid contactors have two roles on a boat.
1, they are a starter motor solenoid contactor
2. battery isolation solenoid contactor, The easiest way to test is to hold you hand against the solenoid and turn the ignition key. You will feel it click in and then you will know what you have,If this is close to your stater motor and one of the heavy cables connects to the starter, it is the starter solenoid contactor.
A thought for later.
On my boat the start battery and the house battery have no connection between them expect for a charge selector.
I flick a switch to send my super powerful (35w) solar charge into the battery of choice. So they are isolated.
I have jumper cables if I ever want to hook up both batteries to start.
I have a charge selector for a 4WD with two batteries. It senses which battery need a boost when the engine is running and feeds into that one automatically.
PS the terminal strip on the left of the solenoid looks in pretty bad shape. Off to Bunnies.
gary
Select to expand quoteAzureF305 said..
Thanks Shaggy and Seamonkey - I think you've connected to a few dots for me. All measurements were made at night with everything turned off, so no charging or load. Im now thinking that its a bit of bodgey wiring in combination with a bad starter battery. The house system is rock solid and checks out fine, so I'm looking at the starter as the fail. Ill do the testing tonight, and the physical sighting tomorrow with better light. I think only one wire from the start neg connected to the house so I can re-run that one to the house- that would solve that problem, and I could probably work out where the other ones are coming from, but i think my biggest issue will be sorting this -
- or do i just sort out the batteries and leave this alone? (pic should be portrait, sorry)
I'm guessing it's the lifting pump but why all the wiring?
The solenoid switch could be set up as a battery combiner, same as is sometimes used by four wheel drivers with a second battery. If that is the case then the pos switch wire will have a switch somewhere, search for it and see if it is switched on. That would explain the strange abilities of the system.
Didn't think of that, Uncle Bob.
Goes to show how important an up-to-date wiring diagram is.
If you fit something electrical, make a diagram and make wires.
then pass it on to the new owner when you sell.
Gary
Select to expand quotewoko said..
Azure, I agree start battery likely to be suss. what charging system/s do you have ? Alternator only - panels - wind ? And I'm guessing both 12v systems ?
Hi Woko, - yep 12-volt systems, both charged by 115a alternator, and both charged by solar(240w) through MPPT controller. Both circuits charge on the solar regardless of master switch positions.Select to expand quoteSeamonkey_H2024 said..
Azure, do you have an automatic charging relay in your circuit?
If this is one, then yes. Otherwise no.
What does this do anyway?
Thanks Gary, your information was great - i had no idea, but I've learnt a lot now.
These solenoid contactors have two roles on a boat.
1, they are a starter motor solenoid contactor
2. battery isolation solenoid contactor
I'm pretty sure it's number 2, starter motor is right over the other side of the engine, and this solenoid is mounted between the two master switches.Select to expand quoteUncleBob said..
The solenoid switch could be set up as a battery combiner, same as is sometimes used by four wheel drivers with a second battery. If that is the case then the pos switch wire will have a switch somewhere, search for it and see if it is switched on. That would explain the strange abilities of the system.
Couldn't find a switch for it, but I think it's what Gary has said above.
Well Shaggy, I went to test the start battery as you suggested, and the result was instant. I didn't need to use a load for testing- as soon as I took the start battery out of the circuit it dropped, nay plummeted to 11.4v in about 20 secs, then overnight dropped further to 10.8v - yep the starter battery was well and truly stuffed . Replaced this evening with a new one, and a new set of terminals.
G'day Azure,
That sucks and is great news all at once! Sucks you had to replace it, but now you have a new battery .
Good on you for chasing it down, replacing an engine battery at the dock is sooo much better than finding out when you're a gazillion miles from anywhere.
I was down the boat yesterday mucking around with the NKE display layout when I noticed a high current alarm on the autopilot page. I repowered and reset everything, and sure enough, as soon as I went to engage the autopilot the same alarm popped up again. Turn off the engine, no high current alarm. Hmm.
A high current alarm to me says my regulator on the alternator has gone kaput. This would explain why I have had to replace a couple of batteries, I think my charging system has been overcharging and eating batteries. I 'll get down the boat today to have a good look in the daylight, I dont know on the Penta 30D if the reg is internal in the alternator or a black box hidden in the wiring somewhere....gotta love tinkering with boats!
Select to expand quoteAzureF305 said..
Hi Woko, - yep 12-volt systems, both charged by 115a alternator, and both charged by solar(240w) through MPPT controller. Both circuits charge on the solar regardless of master switch positions.
What does this do anyway?
here you go:
marinehowto.com/automatic-charging-relays/
^^^ that's probably the best site for expert info about batteries and electrics BTW ^^^
Select to expand quoteAzureF305 said..woko said..
Azure, I agree start battery likely to be suss. what charging system/s do you have ? Alternator only - panels - wind ? And I'm guessing both 12v systems ?
Hi Woko, - yep 12-volt systems, both charged by 115a alternator, and both charged by solar(240w) through MPPT controller. Both circuits charge on the solar regardless of master switch positions.Seamonkey_H2024 said..
Azure, do you have an automatic charging relay in your circuit?
If this is one, then yes. Otherwise no.
What does this do anyway?
Thanks Gary, your information was great - i had no idea, but I've learnt a lot now.
These solenoid contactors have two roles on a boat.
1, they are a starter motor solenoid contactor
2. battery isolation solenoid contactor
I'm pretty sure it's number 2, starter motor is right over the other side of the engine, and this solenoid is mounted between the two master switches.UncleBob said..
The solenoid switch could be set up as a battery combiner, same as is sometimes used by four wheel drivers with a second battery. If that is the case then the pos switch wire will have a switch somewhere, search for it and see if it is switched on. That would explain the strange abilities of the system.
Couldn't find a switch for it, but I think it's what Gary has said above.
Well Shaggy, I went to test the start battery as you suggested, and the result was instant. I didn't need to use a load for testing- as soon as I took the start battery out of the circuit it dropped, nay plummeted to 11.4v in about 20 secs, then overnight dropped further to 10.8v - yep the starter battery was well and truly stuffed . Replaced this evening with a new one, and a new set of terminals.
I'm still thinking about it...
www.sidewinder.com.au/page166a.html >>Here is a LED diagnostic chart, could be handy to print off for the boat.
Depending on your wiring and battery health, your batteries can stay in // while the engine is off an isolated. Does your solar relay feed into your main? and is your acr on the battery side of your main/aux isolating switches? Maybe check for any leakage on your main batteries and that your solar relay is working correctly.
Hope this isn't confusing you, just throwing some ideas around.
Well it's all back in place and everything seems to be working correctly Yay!
Shaggy - I noticed when I took the old start battery out that the sticker showing the installation date (which was hidden under the hold down strap) showed February 2007 !, so I think I got a pretty damn good run out of it!!
Fishmonkey - thanks for the link. Definitely a good read, and made me feel secure that the ACR (now that I know what it is!) is in the right place and doing what it's supposed to do. Very comforting.
Seamonkey - brilliant, thankyou!, the diagnostic chart is exactly what I needed.
Depending on your wiring and battery health, your batteries can stay in // while the engine is off and isolated. Yep, but only when it needs to, now i know the acr is working properly.
Does your solar relay feed into your main? Yes
and is your acr on the battery side of your main/aux isolating switches? Yes
Maybe check for any leakage on your main batteries checked, and nothing suss.
and that your solar relay is working correctly. Yes, checked all the parameters, working correctly
Not confusing , actually clarifying. Now I don't have to panic when the damn led goes off - which is what I've been doing
- with the chart I can see it's doing exactly what it's supposed to, and that makes me feel a whole lot better.
I put everything back together pretty much the way I found it, apart from replacing old connections etc, I figured it's been working properly for the last 13 years so don't change it.
One thing I did find a bit weird was that with the start battery in // with the rest of the system, when the fridge cuts in, there is no draw on the start battery. Don't quite know how this has been achieved, but I think it's pretty cool.
A big thankyou to everyone who has contributed - the 'black art' of my 12v systems have become a bit less black, and a bit more grey.
I love the SB forum! What would have taken me weeks to sort out on my own has been solved in just days with the combined brains trust! Gotta love that
Select to expand quoteCharriot said..
might help
G'day Charriot.
The formula on the right is not quite right - although I don't think it is as relevant to us yachties as the other two.
You can't work out voltage like that - "R" is resistance, so if you want to find the resistance of a piece of equipment or circuit, you would use that formula: volts divided by amps.
Resistance is measured in Ohms.
So say 12v / 2 amps = 6 ohms.
Azure
Start making a wiring diagram with wire colours.
If you do it job by job you will eventually end up with something really useful.
Glad you are back in action.
Gary
Excellent idea Gary, but lots of wiring in either black or red. I'll probly have to do a trace and tag each circuit, but like Troubadour says - a good job for winter!
Select to expand quoteAzureF305 said..
One thing I did find a bit weird was that with the start battery in // with the rest of the system, when the fridge cuts in, there is no draw on the start battery. Don't quite know how this has been achieved, but I think it's pretty cool.
the main purpose of the ACR is to isolate the batteries unless there is a charging source available that is capable of handling all the current loads and charging both batteries at the same time.
how this works in practice depends on where your loads and charging sources are connected. usually your fridge and main solar panels would be connected to the house battery. if your system is setup to suit the way the ABR Sidewinder ACR is designed to work, then the alternator is connected to the start battery.
Select to expand quoteNowandZen said..Charriot said..
might help
G'day Charriot.
The formula on the right is not quite right - although I don't think it is as relevant to us yachties as the other two.
You can't work out voltage like that - "R" is resistance, so if you want to find the resistance of a piece of equipment or circuit, you would use that formula: volts divided by amps.
Resistance is measured in Ohms.
So say 12v / 2 amps = 6 ohms.
i think the formula is as intended, but whoever made the diagram typed the wrong heading in the table, and meant to put "To find resistance" in the third column...
